When an American soldier received the letter "Dear John" it meant..

the end of the relationship

birth of a child

a military discharge

decease of a spouse

A Dear John letter is a letter written to a man by his wife or romantic partner to inform him their relationship is over because she has found another lover. While the exact origins of the phrase are unknown, it is commonly believed to have been coined by Americans during World War II. "John" was the most popular baby name for boys in America.